High density compressible iron powder and process



Patented Aug. 7, 1945 HIGH DENSITY CODIPRESSIBLE IRON POWDER AND PROCESSJoseph E. Drapeau, Jr., Calumet City, 111., assignor to The GliddenCompany, Cleveland, Ohio, a

corporation of Ohio No Drawing. Application February 6, 1943, Serial No.474,976

10 Claims.

This present invention relates to an improved type of iron powder and tothe method of making the same.

Iron powder heretofore used for the manufacture of pressed shapes hasgenerally been of reladuced on pressing. High density iron powders arenot suitable for many purposes since the milling necessary to producethe high density results in particles of nearly spherical shape havingsmooth surfaces, which on pressing do not give the desired greenstrength in the product when using practicable pressures. High apparentdensity, however, is desirable in order to reduce the size of the diesused in the pressing operation, and in order to increase the size andnumber of products which can be produced by this economical method offabrication.

It is accordingly an object of the present invention to produce animproved high apparent density iron powder.

Another object is to provide a process for producing an improved ironpowder having a .relatively high apparent density.

Iron powder as ordinarily produced by reduction of iron oxide has arelatively low apparent density due to the porous and irregular natureof the particles. The grains making up the particles of such a powderare of a poly-crystalline nature which results in so-called slip planeinterference in the movementof the solid metal under pressure. It hasbeen found, however, that if a high density powder, the particles ofwhich have a mono-crystalline structure, is first subjected to a flashoxidation to produce a surface oxide film,

and this film is then subjected to a low tempera- 0 ture reduction for ashort time that the desired objects may be accomplished.

The surface oxidation and reduction results in a new iron .powder ofhigh apparent density, the particles of which are of mono-crystallinestructure and have an outside surface of a spongy porous nature. Themono-crystalline powder, whether of pure ferrite, or iron containingdissolved or dispersed carbides ofgranular pearlite lends itself toeasier slippage of planes bringing '50 about a marked interlocking ofthe metal powder during pressing. The porous rough surfaces also assistin the interlocking of the particles. The powder is thus one whichproduces relatively high green strength on pressing with the advantageof u the high apparent density that would not be possible with anuntreated iron powder.

In carrying out the invention porous polycrystalline iron powderproduced by the low temperature reduction of iron oxide may be firsttreated according to United States Patent No. 2,181,123. The porousmaterial is first subjected to a rather severe milling operation toproduce a powder of relatively high apparent density, preferably in amill of the hammer type, This milling reduces the powder particles toapproximately spherical shape having smooth surfaces, The milled powderis then subjected to a heat treatment in a reducing atmosphere, or underother non-oxidizing conditions at a temperature at which normal graingrowth occurs to convert the poly-crystalline material to themono-crystalline state. As mentioned in Patent No. 2,181,123, thetemperature of this heat treatment will be in the range of 900 to 1650F.

The mono-crystalline material is then subjected to a short oxidationwith air or in air with steam to produce a surface oxidation, wherebythe powder becomes dark gray or reddish in color. The amount ofoxidation is not great, and the oxide film may only be five or sixmolecules in thickness. Since a relatively high apparent density isdesired the amount of oxidation will not be such that the apparentdensity of the final powder is less than desired. The partially oxidizedmaterial is then reduced with hydrogen, CO or other suitablenon-carbonizing reducing material at a temperature low enough topreventsintering, preferably at about 600 C. This reduction will takeplace in a very short time. The material thus produced is then cooledunder nonoxidizing conditions, given a light tumbling operation andscreened. The particles of the material so produced have spongy surfacesof pure ferrite since the surface treatment removes any carbides whichmay have been originally present in the surface.

Alternatively the conversion from the polycrystalline state to themono-crystalline state may be accomplished after the oxidation step, byhaving the conditions for the reducing action such that grain growthoccurs.

The powder thus producedshows a decided improvement over an iron powderof the same chemical analysis and with a mono-crystalline structure, buthaving smooth surfaced ball-like particles. The powder so produced hasan improved green strength and also sinters more rapidly than theuntreated powder. The pressed and sintered product is thus of greaterstrength. The advantages of high apparent density in the moldingoperation, however, are retained. An apparent density of from 2.3 to 2.6is desirable but heretofore such a powder did not produce the desiredgreen strength. The powders produced by the present invention may havesuch an ap-' parent density, are highly compressible and produce highgreen strength and rapid sintering pressed products.

It is to be understood that it is within the spirit of the presentinvention to use as a starting material any iron powder having a.relatively high apparent density, and bringing about a roughening of thesurface by a surface oxidation followed by reduction, it being kept inmind that if such starting material is poly-crystalline in nature,conversion to the mono-crystalline state will be efiected.

The temperature of the heat treatment of the iron powder is preferablyabove 1000 F. but below the point at which sintering takes place. Themaximum is about 1650 F. with a minimum of 900 F. At or above therecrystallization point there is a change from alpha iron, a bodycentered cubic crystal, to gamma iron, a face centered cubic crystal.

I claim:

1. An iron powder composed of particles of mono-crystalline structureand having thin, porous, spongy surfaces of substantially uniformthickness.

2. An iron powder having an apparent density of from 2.3 to 2.6 andcomposed of particles of mono-crystalline structure and having thin,porous, spongy surfaces of substantially uniform thickness.

3. An iron powder characterized by having a relatively high apparentdensity composed of particles of approximately spherical shape ofmono-crystalline structure, and having thin, porous, spongy surfaces ofsubstantially uniform thiclmess.

4. An iron powder composed of particles of mono-crystalline structurehaving cores containing dissolved carbides and thin, porous, spongysurfaces of pure ferrite iron of substantially uniform thickness.

5. The process of producing an improved iron powder which comprisessubjecting an iron powder of relatively high density composed ofparticles of mono-crystalline structure to a surface oxidation and thento a reducing treatment to reduce iron thus oxidized at a temperatureinsufficient to cause substantial sintering.

6. The process which comprises subjecting an iron powder composed ofsubstantially spherical particles of mono-crystalline structure andhaving smooth surfaces first to a surface oxidation, and then to areducing treatment sufiicient to reduce the thus oxidized iron at atemperature insumcient to cause substantial sintering.

7. The process of producing an improved iron powder which comprisessubjecting an iron powder to a surface oxidizing treatment, then to areducing treatment to reduce the iron thus oxidized at a temperatureinsufficient to cause substantial sintering, said process being furthercharacterized in that the starting material is polycrystalline and thepowder is subjected to a treatment to convert the same tomono-crystalline structure at some stage in the process.

8. The process of producing an improved iron powder which comprisesconverting a powder composed of particles having a polycrystallinestructure to particles of mono-crystalline structure, surface oxidizingthe mono-crystalline particles and then reducing the thus oxidizedsurface film at a temperature insufiicient to cause substantialsintering.

9. The process of improving iron powder having a polycrystallinestructure and relatively low apparent density and composed ofirregularly shaped particles which comprises first subjecting such apowder to a milling operation to produce a powder of relatively highapparent density, the particles of which are approximately spherical inshape and have smooth surfaces, subjecting the thus milled powder to aheat treatment to develop a mono-crystalline structure, surfaceoxidizing the mono-crystalline material, and then reducing the surfaceoxide film at a temperature insuflicient to produce substantialsintering.

10. The process of improving iron powder having a polycrystallinestructure and low apparent density and composed of irregularly shapedparticles, which comprises subjecting such a powder to a millingoperation to produce a powder of relatively high apparent density, theparticles of which are of approximately spherical shape and have smoothsurfaces, surface oxidizing the particles, subjecting the surfaceoxidized material to a heat treatment under reducing conditions toreduce the surface oxide film and to convert the polycrystallinematerial to monocrystalline, the temperature being insuficient to causesubstantial sintering.

JOSEPH E. DRAPEAU, JR.

